texts: In a reflecting telescope (Cassegrain), is there a difference between using a concave or convex secondary mirror?

texts:
These are two diagrams of Cassegrain reflecting telescopes.

One uses a concave secondary mirror, and the other uses a convex one.

Is there any difference between the two?

For example, does one cost more? Does one produce a better image?

Thanks!

===

EDIT: These images came from:

http://astro-canada.ca/les_telescopes-reflecting_telescopes-eng
http://www.a-levelphysicstutor.com/optics-telescs.php

===


texts: Is it fair to describe a solar flare as a big lightning strike on the sun?

texts:
Solar flares and lightning strikes are both kinds of electric discharges.
A lightning strike is a sudden discharge of electrical energy between an electrically charged cloud and another object, like the ground or another cloud.
A solar flare is a sudden discharge of electrical energy between two electrically charged regions, or "clouds" in the atmosphere of the sun.
Duration
A lightning strike lasts about 0.2 seconds, and it is made up of a series of short strokes of about 60 to 70 microseconds each. It produces an electromagnetic pulse (EMP) that consists of electromagnetic radiation across the spectrum from radio to gamma waves.
A solar flare lasts on the order of minutes to tens of minutes, and is also comprised of series of short bursts. This video of the Cinco de Mayo solar flare shows the way that the solar flare is comprised of multiple short discharges of electromagnetic radiation, which can be seen in the visual spectrum and also detected in other wavelengths like radio, ultraviolet, and x-ray, and gamma.
Amount of energy

Large lightning strike: 109 joules
Large solar flare: 1025 joules

Some other gifs of solar flares

Berkely.edu
Spaceweatherarchive.com
Cosmosmagazine.com

Relevant links

Lightning - Wikipedia
Solar flare - Wikipedia
Full-spectrum EMP

Related questions

What triggers solar flares?
What are the main differences between solar flares and coronal mass ejections
Can lightning occur in stars like the sun?
Typical wavelength of solar flare

My question
I'm also wondering if a solar flare is an electrostatic discharge. My main question is:
If we were to make an oversimplification about solar flares, would we say that they are big lightning strikes on the sun?


texts: How are the element abundances calculated for a meteorite in the Hydrogen log10 scale?

texts:
In astronomy, solar abundances are often calculated or tabulated per $10^{12}$ H atoms. I understand that in case of the Sun this can be done because H atoms are in majority or are present for a scale comparision.

How can the same element abundance be calculated for an example meteorite which does not have any volatile gas such as H? In particular, I am referring to the table 1 of http://astro.uni-tuebingen.de/~rauch/TMAP/grevessesauval2001.pdf. It lists Solar abundance and lists meteorites abundance as a comparision. How have they calculated the element abundance for a meteorite in the $log_{10} \epsilon_{H} = 12$ scale?

Finally, I have a sample rock which has its's composition given in Weight $\%$ of various oxides such as $SiO_{2}, MgO, FeO$ etc. I have to convert these into elemental abundances in $log_{10} \epsilon_{H} = 12$  scale. How should I do this?

Thanks in advance!


texts: How can we detect water on Mars-like exoplanets?

texts:
According to data from Curiosity, Mars' dust holds about 2% water by weight

This wasn't previously detected, so the impression we have had of Mars being incredibly dry may need to be altered. Okay it is still very dry, but there is potentially extractable water in amounts usable by colonists.

But this sort of thing would be good to discover before going to another planet, so how can we detect this sort of thing without actually going to the surface and heating up the local dirt?


texts: In 31.5kyr, Epsilon Eridani and Luyten 726-8 will be < 1ly close; but how distant from the Solar system?

texts:
Greetings! Based on Wikipedia and more precisely this paper, it is said that, in about 31,500 years, the stars Epsilon Eridani and Luyten 726-8AB will "meet" at a very close distance (less than 1 light year); I found that anecdote very interesting in the context of some SF writings I'm trying to do (not in English, quite obviously!) to bring some "subtle consistency" in it, but I was unable to either answer nor calculate with my very little knowledge of astronomy this simple question: to what distance will those stars be from the Solar System in 31,500 years?

Thank you very much for your help!

P.S.: oh, a friend of mine told me that my question could lack of useful information, such as the current distance of each of those stars from the Solar System, so here there are:

Current distance from Epsilon Eridani to Solar System: 10.5ly
Current distance from Luyten 726-8 to Solar System: 8.7ly
Other info about Epsilon Eridani: declination 9.46° south of the celestial equator (no idea if this can help) | Right Ascension 03h 32m 55.84496s | Radial velocity +15.5 ± 0.9 km/s
Other info about Luyten 726-8: declination –17° 57′ 01.8″ | Right Ascension      01h 39m 01.54s | Radial velocity +29.0 km/s

[EDIT]
As suggested in the comments, I added the Right Ascension and Radial Velocity for those two stars and I'm looking for the "Velocity vectors" (even if I have no idea what this means)

[EDIT 2]
Added links to the Extended Hipparcos Compilation (XHIP) Epsilon Eridani and Luyten 726-8AB pages with all sort of info I don't understand, but there are things such as "Heliocentric Velocity" on 3 axis, maybe it can help. (Got the XHIP thing thanks to the following thread: Position and velocity vectors of nearby stars?)


texts: How ready are we to deal with a asteroid/comet/giant-meteor on a collision course with Earth?

texts:
I keep hearing astronomers saying there are observatories scanning the skies all day and night looking for near-Earth objects, extrapolating their trajectories.

My questions are

If we were to get the news of an on-coming object, how early we will find out or how much time we will have?

Currently, people don't even think about it; Governments are cutting spending in their space exploration programs, let alone deflecting an impact-er.

Under certain circumstances, would the notice from the watchdogs be enough for us to build something that would prevent the loss?


texts:
It’s well-known that the axial tilt of the Earth (with respect to the ecliptic) is about 23.4 degrees. However, two angles is needed to specify the orientation of any rigid body, so it’s unclear to me exactly how Earth is oriented with respect to its orbital plane.

To be more precise, consider the plane that satisfies these two conditions:
1. Perpendicular to the ecliptic plane
2. Contains the line connecting the Sun and the Earth when Earth is at perihelion (Jan. 2).

(Basically, this plane cuts through the orbit of the Earth such that for half the year Earth is one side and for the other half Earth is on the other side.)

Questions:
1. When the Earth is at perihelion, what is the angle between its rotational axis and the plane described above? I would guess this angle is very small from the images of Earth’s orbit. Is there a technical term for this angle?
2. Does the direction of the axis remain approximately the same throughout its entire orbit?

Please ignore precession for this question.


texts: Any method to apply inverse TPoint model?

texts:
I have an automated telescope that I have written some software for. I am currently using TPoint to compensate for some of the irregularities of my system.

However with TPoint you apply an offset (based off of the given coefficients and the telescopes pointing position) to the target and that is where you point to acquire the target in your viewfinder.

Does anyone know how to invert this process, such that given your known telescope point position, you evaluate your "true" pointing position.

There are 6 parameters:

IA   = Azimuth axis index error (encoder offset).
IE   = Elevation axis index error (encoder offset).
NPAE = Az/El axis non-perpendicularity.
CA   = Elevation axis / Pointing axis non-perpendicularity.
AW   = E-W misalignment of Azimuth axis.
AN   = N-S misalignment of Azimuth axis.

And the offset equations come out to be:

  azimuth_Correction = (-IA) + (-NPAE * Tan(boundedElevation)) + (-CA /
  Cos(boundedElevation)) + (-AN * Sin(azimuth) * Tan(boundedElevation))
  + (-AW * Cos(azimuth) * Tan(boundedElevation));

and

  elevation_Correction = (IE) + (-AN * Cos(azimuth)) + (AW *
  Sin(azimuth)));

Where: boundedElevation is the "capped" value so that the elevation value gives the correct value for Tangent. This is:

MAX_TANGENT_Q1 = Atan(10);
MAX_TANGENT_Q2 = (Atan(-10) + PI);

The TPoint can be found documentation HERE


texts: Upgrading telescope from 5" to 8" or 10"?

texts:
I currently have Celestron Nexstar 130STL.  I am trying to upgrade my telescope to next one.  I examined Meade 8"ACF Goto telescope, but I was not too thrilled about it.  It gave me better view, but the details were not too different from my 130mm.
Maybe my expectation was too big.  I have an option of getting Orion 10" dob motorized alternatively.  I am slightly concerned about the size and the portability.

I would appreciate any recommendation, thanks!


texts: In layman's terms, why do we put the "time" in space-time?

texts:
What does "time" mean in the term space-time? I am trying to understand why anything that travels through space, from a proton to massive galaxies, is always referred to as traveling in space-time, not just space. Does this have to do with time being relative?

I understand that 3d space and time together constitute a space-time.

Can someone illustrate with a scenario to point out the need to describe things in terms of space-time.


texts: Are there more stars in the universe than grains of sand in the Earth?

texts: My ten year old daughter asked me this question at breakfast and I initially said yes, but on second thought I was also puzzled.


texts: Can weak gravitational lensing or microlensing-induced wavefront distortion limit resolution of absurdly large aperture telescopes?

texts:
This is a theoretical question.

This answer to the question If we had the right technology could we see a distant star in detail? (presumably space-based) primarily addresses the scaling of diffraction-limited resolution with aperture $D$. For existing and near-future telescopes this is assumed to follow $\lambda/D$.

Imagine a future optical or radio interferometric telescope of absurdly large effective aperture or baseline, say AU or even larger, operated similarly to the Event Horizon Telescope (local downconversion and timestamping, interferometry performed offline)

Could weak gravitational lensing effects or gravitational microlensing result in wavefront distortion that if not corrected, would result in degraded resolution? Note I'm not asking about distortion of the field, but instead a limitation to the resolution; degraded point spread function.

If so, would this problem be amenable to correction in the same way that atmospheric and ionospheric effects (visible and radio) are dealt with? Would it generally be static on the timescale of the observation?


texts:
What is the length of the apogalacticon and perigalacticon of the Sun and Milky Way?  The general terms seem to be apoapsis and periapsis.

My greatest efforts at Googling have failed miserably.  If you can provide references as well, please do!


texts: Converting FITS (NuSTAR) coordinates to WCS

texts:
I'm trying to convert some pixel coordinates I have into WCS coordinates, ideally into a WCS region for use in some further analysis.

So far I've been able to load and parse a NuSTAR FITS file, do some analysis to make my selections and get the pixel coordinates for the image. For example, after my analysis I would end up having selected a rectangle of x values from pixels 480 to 518, and y values from 478 to 516.

After that I tried to use the astropy WCS module to convert them, but it doesn't seem to find the required data in the header to actually do the conversion and just says that pixel 480 gives coordinate 480. That or I'm just doing something wrong.

So I looked through the FITS header myself, found that some keys (TCRPX, TCRVL, TCDLT) give the reference pixel, reference pixel degree coordinate and pixel axis scale values.

But now I'm really stuck since what I need to get is a region file like those produced by ds9, in the format of:

# Region file format: DS9 version 4.1
global color=green dashlist=8 3 width=1 font="helvetica 10 normal roman" select=1 highlite=1 dash=0 fixed=0 edit=1 move=1 delete=1 include=1 source=1
fk5
circle(19:58:21.593,+35:11:57.46,10")

To then run this through the nuproducts tool.

TL;DR: Does anybody know how to convert the NuSTAR FITS physical pixel coordinates to FK5 coordinates?

EDIT: Wrote some code to extract the coordinates. It converts the pixel to degree fine, but converting the FK5 $\alpha$ angle to sexagesimal doesn't work correctly for... some reason.

EDIT2: Everything works now, here's a notebook where I run through it all in case anybody runs into a similar problem. I go through extracting the right-ascension and declination in degrees from the pixel coordinates given by the FITS file, then converting these degrees to sexagesimal (h:m:s) angles. From my tests it seems to work quite well, results are a tad off  due to what I assume are some floating point arithmetic errors.


texts: Do elliptical galaxies look similar to globular clusters?

texts: If we got close enough to an elliptical galaxy would it look similar to a globular cluster?


texts: Is it possible for a black-hole to easily stretch or turn any hard object like (Diamond , Gold bar , Platinum bar , Brass) into spaghettification state in just a seconds or minute once it gets near nor reached the black-hole's event horizon??


texts: Do far away stars that we observe today still exist?

texts:
I don't know much about the astronomy or the universe but enjoy reading articles from time to time as they pop up. Just finished reading an article on CNN that discussed how a South African telescope has discovered hundreds of galaxies, some 200 million light years away.

This got me thinking: Are these galaxies and or some of their stars we see in telescopes still there? Let's say STAR X is confirmed to be 200 million light years away. What if it completely exploded 1 million years ago? Wouldn't we need to wait 199 million years before we could detect that?


texts: Name and language of symbol in astronomy book

texts:
I've been working on a programming project involving Peter Duffett-Smith's Practical Astronomy With Your Calculator (3rd Ed.), which has a large number of mathematical/astronomical formulas in it. There's a part at the end of the book for symbols and abbreviations, which contains this:

After doing some Google-fu, I was unable to find the name of the symbol. Would anyone smarter than me (which should be quite a few of you... heck, quite a lot) know what this is? Hopefully there's Unicode for it...


texts:
From my research it doesn't appear to be from tidal locking with the sun.

Is there somewhere I can start reading about the probable causes?


texts: Why aren't the farthest objects close to each other?

texts:
The last years we find some objects like this : MACS0647-JD which are more than 13 billion light-years away from us.

This means that the picture we get was created only some million years after the Big Bang.

Why can we see objects like this in all directions? Shouldn't those objects be close to each other?


texts: Does the focus of terrestial telecopes need adjustment for the distance of astronomical objects?

texts:
My rudimentary experience of photography and telescopes suggests that with commonly affordable optics any object beyond the range of hundreds to thousands of feet will be in adequate focus at the proper "infinity" position. For example the advice here for overcoming difficulties in seeing the focus of a star on a DLSR screen to instead focus on a brighter and distant city light: https://photo.stackexchange.com/a/23973.

However an interlocutor recently offered this advice (collated from a series of comments):

Apparently you haven't actually used a camera or a telescope. The focus adjustment is there for a reason. Any astronomer will tell you focus changes are needed between stars, stars farther away,  planets, and the Moon. ... If focused on the Moon, you need to refocus slightly when looking at stars. When looking at Jupiter you need to refocus if the target is the Moon. Close stars vs far away stars, the same thing, a need to refocus. ...  To the tune of: Jupiter and it's moons, which are close to Jupiter , need focus adjustment between them. Either Jupiter or it's moons will be in focus. Ask any astronomer. That's a difference and it's well past the supposed "infinity focus" point.

I wouldn't be wholly surprised if retargeting a telescope could require a focus tweak to correct for slight changes of the mirror distances (etc), or from thermal shrinkage across a cooling evening, but such effects would probably be invariant across objects, whereas as described in that quote targetting back on the original object would presumably require the focus to be restored to the original setting.

Is there a distance-dependent component for focus in astronomical observation, and if so what sort of equipment does it affect in practice?


texts: what is the current explanation for the formation of cosmic voids?

texts: Why aren't galaxies distributed evenly in space, but instead form those sponge-like structures, with huge voids between filaments?


texts: How much higher is the radiation around Jupiter than at Chernobyl, after the explosion?

texts:
I suppose the radiation levels are higher at Jupiter, but how much? Would the Juno probe have been able to "operate" on the Chernobyl roof in $12\,000\, {\rm Roentgen/hr}$?
After watching the short series Chernobyl, I was curious how the radiation levels compare with that around Jupiter (generally), and what the Juno probe in particular have to withstand, but I couldn't directly compare the numbers I found.
In the Chernobyl accident, the reactor explosion causes pieces of graphite to fall on the roofs of the reactor building, and they describe that they can't push them off with remote-control machines because the radiation damages the circuits.
In the series, the value given for the most dangerous roof is $12\,000\, {\rm Roentgen/hour}$.
The Juno probe's radiation vault is a titanium box with ${\rm 1.75cm}$ thick walls, and the article says

But this orbit would still expose an unprotected probe to a radiation
dose of more than 20 million rads [...] over the course of its mission.

The mission was planned for 14 Earth months but the probe only spends short periods of time around Jupiter.
Perhaps someone has an idea how to fill in the gaps and help to make a comparison.


texts: Jupiter has a minimum brightness that is brighter than stars. Is the reason an “airy disc”, concentric rings as a diffraction effect, is seen around stars and not Jupiter because Jupiter has an angular size that is thousands of times larger? With similar reasoning, if the field of view was scaled up a thousand fold, would the optical effects be similar to that around stars?


texts: Can I get help getting image of Celestial Body at RA 2.220 and Declination +15.59?

texts: My mom had a star named for my dad who died a few years back.   I told her that I'd try to find an image of that star and send it to her.  My dad's Birthday was 3/31/1931 if that helps.  Would love some help with this.  Thanks so much, in advance.  :-)


texts: What happens if a black hole is produced inside another black hole?

texts:
Similar question here.

The question I linked got me thinking.

It is known that a black hole doesn't let anything escape its event horizon, not even light. Thus, naturally, anything that passes the event horizon would get "eaten".

Now, assume a supermassive black hole(small Roche limit, large event horizon) was in the path of a high eccentricity, massive, dying star. Assume the star is travelling in a straight line at 5% the speed of light, and the black hole is stationary. The star would be swallowed by the black hole, and also swallowed intact. Now, the star goes supernova inside the black hole, but since its mass is very high, it too, creates a black hole. What would happen?

Would the supermassive black hole gain the mass of the star and things would go as normal for an outside viewer(everyday common things: black hole swallowing objects)? Would the black hole inside the larger one pull the larger one in? Or something else?

Thanks!

Max0815


texts: LIGO gravitational wave chirp signal frequency

texts:
This question is about the LIGO gravitational wave signals' frequency. The signals start from about 35 Hz to peak at about 250 Hz, giving evidence of gravitational waves. The question is about the exact frequencies the waves' detection started to the exact peak of the wave.

It seems the LIGO graphs show the peak is a little
above  250 Hz. Are there tables published by LIGO that put the start and peak frequencies as 35 to 250 Hz? The graph in the paper by Abbott, et al shows the start and peak (the start is around 36-37  Hz, the peak seems to be at around 256 Hz) - are there exact numbers (tables) for the start and peak, that one can reference?


texts: How many times can iron be detected for the first time in an exoplanetary atmosphere?

texts:
I see today that there's a story about the first detection of iron in an exoplanetary atmosphere, in this case in the atmosphere of the ultra-hot Jupiter KELT-9b.

I remember there being a story a couple of months ago about the detection of iron rain in an exoplanetary atmosphere, the example being another ultra-hot Jupiter, WASP-76b.

It doesn't seem plausible for both of these to be the first detection of iron in an exoplanetary atmosphere. So which one is it?


texts: Where do Neutrinos Go if They Don't Interact?

texts: It is said that neutrinos can travel for billons of years at almost the speed of light without interacting with anything. That being the case,what do they do - is it possible they could orbit the universe indefinitely? They certainly can't leave it.


texts: How do the orbits of Nu Scorpii and AR Cassiopeiae work?

texts:
I apologize for the way this question is worded but I don't think I know the proper verbiage. Basically, I want to know how the stars orbit one another in the two septuple star systems. For example, are there two massive stars in the middle that have the five remaining stars orbiting similar to the planets of our system; does one of the orbiting stars have one or more smaller star-"moons;" maybe there are stars trapped in one of the Lagrange Points. I hope that clears up the question.

I am aware that the orbits of these systems may not be known/understood. If that is the case, then I would appreciate the orbital relationships between the stars in the highest-multiple star system. If that is confusing, I mean, if we know the orbits for the stars in a sextuple system; if not then a quintuple... etc.

Finally, as an added bonus, I would appreciate being told the correct verbiage. Thank you, much.


texts: How do you determine which star, between two, is more luminous?

texts: For example, if star 1 is 4x bigger than star 2 but both are the exact same temperature how do you determine which is more luminous?


texts: How to derive density and mass from the pseudo isothermal model for mapping the density profile for dark matter?

texts:
I recently read about dark matter haloes and ways to profile their density. I came across this model called the pseudo isothermal model for the halo and I wanted to know how you would model the density profile of dark matter when you are given just the rotation velocity? I also wanted to know how you would find the mass from the equation?
Also, are there some resources I can use in order to get a good grasp of the concept?


texts: Why are radio telescopes shaped so differently than optical telescopes?

texts:
Why are radio telescopes typically only a dish with a receiver above it, while optical telescopes have a primary, plus secondary and sometimes even a tertiary mirror?

In other words, why do radio telescopes have only one reflector, while optical telescopes have up to three or more?

The same wave phenomena, such as focusing, should apply in both cases. So I don't understand why the geometry would be radically different. You could just replace actual mirrors with a convenient solid, such as plastic, that reflects radio waves just as wells as mirrors reflect optical waves.


texts: Will the Sagittarius A* Black Hole eventually swallow the entire Galaxy?

texts:
In one of his interviews, Sir Roger Penrose mentioned that when the Milky Way and the Andromeda galaxies eventually "collide" and merge into one, their super-massive black holes will also eventually merge. He further mentioned that the newly formed super-massive blackhole at the centre of the new galaxy will eventually "hoover out" all matter within the galaxy.
I recall reading that this is generally the case with super-massive black holes at the centers of galaxies: they will eventually "eat out" all matter within their galaxies.
Question: why don't the stars and other objects within the galaxies just keep rotating around these black-holes, like they do now? Why will they eventually be swallowed by the black hole's gravity pull?


texts: Electrical potential difference between planets

texts:
In dealing with electricity, we usually refer to Earth as the electrical neutral or ground. Is there any evidence that other planets' ground is at the same electrical potential level as the Earth's?

If somehow, there was a lossless conductive connection between two planets, would current flow accros It? Would this be theoretically a source of energy?


texts: Is this render of a ringed planet's shadow accurate?

texts: I've noticed that when it's the equinox on this planet I'm making, the sun seems to cast a double shadow on the planet. How come the planet's ring seems to make two shadows with a space in between, instead of one "line" shadow. Is this accurate to real life, or a quirk of the rendering engine I'm using?


texts: Is the list of the most abundant elements in the universe a fact?

texts:
I've heard  Neil deGrasse Tyson giving a short list of the most common elements in the universe on a video, which went on something like:

hydrogen
helium
oxygen
carbon
nitrogen
etc

As I understand astronomers use spectroscopy to determine an object's chemical composition.
My question is how can they determine the most common elements for certain, when most of what we see is a just a fraction of the stars, nebulas and an even smaller fraction of planets out there?


texts: Exploiting symmetry in the axisymmetric Jeans equations

texts:
I'm solving the axisymmetric Jeans equations to determine the initial conditions for an exponential disk inside an NFW halo. The density profiles of the two components are
\begin{equation}
\rho_d(R,z)=\rho_{0,d}\exp\left(-\frac{R}{R_0}\right)\exp\left(-\frac{|z|}{z_0}\right)
\end{equation}

\begin{equation}
\rho_h(r)=\frac{\rho_{0,h}}{r/r_s(1+r/r_s)^2}
\end{equation}

Due to the axial symmetry, for the meridional velocity dispersion we have
\begin{equation}
\sigma^2(R,z)=\frac{1}{\rho}\int_z^\infty \rho \frac{\partial \Phi}{\partial z'}dz'
\end{equation}

As the galaxy is symmetric about the disk plane ($z=0$), we would expect the distribution function at a distance $h$ above the disk plane to be identical to that at $z=-h$; in other words, if we flip the galaxy, the only difference will be a change in direction of particle motion, but not speed or dispersion.

Therefore, can't we replace the $z$ in the limit with $|z|$?


texts: Sensitivity of calculated orbital elements to observational errors

texts: These days, we have some very precise ways of making measurements, but I'm sure it wasn't so in Kepler's day. So I am wondering how astronomers of that time could make such accurate determinations of planetary orbits, given the likely limitations of the available instrumentation. To put the question another way: How does each added (or lost) digit of precision in the measurement affect the accuracy of the calculated orbital elements? Is there significant compounding of errors due to the nature of the calculations, or is it essentially one-for-one? How much accuracy (to how many digits of precision) must the "three observations" be in order to determine the orbital elements to a given precision? How does that precision degrade over time? If, for example, we wanted to prepare an almanac for Neptune that will be accurate to say, one second of arc in our sky after 100 years?


texts: Convert coordinates between RA/dec and WGS 84 (SRID=4326)

texts:
I've imported the HYG Database from http://www.astronexus.com/node/34 and now I need to convert all RA/dec stars positions to fit in a WGS 84 coordinates (SRID=4326) PostGIS map (-180 to 180, 90 to -90).

BayerFlamsteed  ProperName  RA            DEC
21Alp And       Alpheratz   0.13976888    29.09082805
11Bet Cas       Caph        0.15280269    59.15021814
88Gam Peg       Algenib     0.22059721    15.18361593
Alp Phe         Ankaa       0.43801871    -42.30512197
18Alp Cas       Shedir      0.67510756    56.53740928
16Bet Cet       Diphda      0.7264523     -17.98668410

Related to:

https://gis.stackexchange.com/questions/84389/spatial-database-containing-coordinates-for-stars-in-the-sky

https://gis.stackexchange.com/questions/2459/what-coordinate-system-should-be-used-to-store-geography-data-for-celestial-coor

EDIT: Ok, I think MerseyViking gives the answer in https://gis.stackexchange.com/questions/2459/what-coordinate-system-should-be-used-to-store-geography-data-for-celestial-coor telling to create a new coordinate system. Can someone give a look and some opinion?

EDIT 2 (After answered): Fantastic!! I need to show the results of whuber and Dieudonné's answer. The first picture I show the query in QGIS. In second picture you can see the real positions in a map. The scale is different - Find Shaula, Nunki and Antares.

This is the conversion method ('H' is for Hyparcus in my database):

insert into stars
    select
        id,
        proper_name,
        ST_SetSRID(ST_MakePoint( (-ra * 15), dec),4326),
        mag,
        'H'
    from hygn
    where (proper_name is not null)


texts: Gamma ray detection for SETI. How far away could we detect an alien nuclear powered rocket?

texts:
I suspect our best bet for finding advanced civilisations is to look for gamma rays characteristic of nuclear/antimatter propulsion.

If an alien civilisation was using something similar to the Orion drive, how close would they have to be for us to be able to detect the gamma rays?

In my mind, this question has two parts:
What are the magnitude/frequency limits on the gamma ray telescopes in orbit? (Specifically Fermi, assuming that has the best detection capabilities of an orbital gamma ray observatory)

What's the magnitude and frequency profile on, say, a 1 megaton nuclear bomb?

I've had a bit of trouble finding these out myself from a casual search, and am wondering if maybe there are military secret issues surrounding the detection of gamma rays.


texts:
I wish I could attach a good quality photograph of this red color moon I'm looking at right now.

I searched on internet but there was no lunar eclipse scheduled to occur on 23rd December 2018

in  HYDERABAD, INDIAN SUBCONTINENT around 7:20pm
Please tell me why the moon is red tonight:) ?


texts: How to get the percentage of the Moon illuminated as a function of time?

texts:
As the title suggests, I need a way to get the percentage of the Moon's disk which is illuminated as a function of time.
There are simple geometrical expressions for the fraction of a sphere illuminated by a collimated coplanar light source, but I need something more physically and geometrically representative of the Sun Earth Moon system.


texts: What are the natural life sustaining resources available in the Jovian moon system?

texts: Hard science fiction writing question.  Premise: what natural resources are present in the Jovian system that would be necessary and desireable to an autonomous, self-sustaining human community?  Water on Europa, for example.  What else? Would like to keep this plausible without a lot of speculation.  Thank you.


texts: How did they make a video of the center of the galaxy, and what is it exactly that's flashing there?

texts:
The ESA video ESOcast 173: First Successful Test of Einstein’s General Relativity Near Supermassive Black Hole includes a clip of images of stars at the center of our galaxy orbiting around SgrA*, a presumed supermassive black hole. This isn't visible light because it's obscured by dust, so it may be radio or long wave infrared, but I don't know.

In the middle, I can see something flashing at whatever wavelength this image has been produced from.

Question:

How are these images obtained, and
what process is it that is believed to be causing that flashing?

GIF made from video at around 02:50:

Six annotated frames from GIF highlighting the flashing that I'm seeing.


texts: What am I looking at here at Google Sky?

texts:
I was looking through the Google Sky map, when an object caught my eye:

I am really curious as to what this is (just a big star? then why does it look so peculiar?)

It's hard to search for online because I don't know which words to use. If anyone can point me in the right directon please?


texts: What is the reason for high lithium concentrations in Sun-like stars?

texts:
In reading the European Space Organisation paper Lithium depletion in solar-like stars: no planet connection (Baumann et al. 2010), several conclusions based on their observations include:

Lithium depletion in Sun-like stars is independent of having planets (an earlier theory), but rather is a function of the star's age.  From the article:

  For solar-like stars, the lithium vs age trends for planet-hosts
  and stars where no planets have been found are statistically
  identical. Thus, the presence of a planet does not influence
  the observed surface lithium abundance.

However, they observed that

  A number of solar-like stars with unusually high lithium
  abundance for their age are present in the field.

and that these stars also have an anomalously low surface gravity.

What is the current theory as to why some Sun-like stars have a relatively high lithium abundance and low surface gravity?


texts: Does the axis of solar system precess during it's motion in the Milky way galaxy?

texts: The plane of solar system makes an angle of 60° with the plane of galaxy. So, the axis of solar system should make 30° angle with the galactic plane. Like the axis of Earth precess over a long period, does the axis of solar system also precess in a similar way? If it does, do we have any estimate of the period of precession?


texts: How to explain the equation 'Altitude of a star at culmination = co-latitude + declination'?

texts:
Altitude of a star at culmination = co-latitude + declination.
I understand that the co-latitude is equal to the altitude of the celestial equator (since it is 90 degrees away from the NCP). I also understand that declination is the height of a star above the celestial equator.
But I am just unsure why you can directly link altitude and declination at the exact point of culmination?
And why is a star reaching its highest point as it crosses the meridian?
thanks for any help.


texts: Number of photons in universe? I'll take number from a star in unit time and extrapolate

texts:
I'll take number per average star, multiple by appropriate numbers , star/Galaxy, galaxies/universe, 14 billion years, and try to figure their location.

I was looking for a rough number and a distribution function.


texts: How do we know the order of the new Trappist-1 planets?

texts:
Using transit photometry astronomers were able to discover Trappist b through h. Based on this question we learn that the naming of planets is based on their distance from their star (b being the closest).

But how do we actually determine the distance from the star? From my understanding with transit photometry we basically just see a shadow against the backdrop of the star. That can tell us the planet exists, but how do we know the distance between the planet and the star?


texts:
Perpetual motion and Big Bang theory seem to allude that the universe can’t last forever because matter and energy are dispensed.
Is it plausible the Universe has infinite matter?


texts: Why are Uranus' satellites named after characters in Shakespeare?

texts:
Oberon, Titania, Ophelia, Puck, Miranda... Uranus' satellites are named for characters in the works of Shakespeare and Pope. Yet the names of Jupiter and Saturn's moon come from the Roman and Greek mythology.
Who decided that the moons of Uranus would be named after English poets? What process led this to come to be?


texts:
I learned that gravity always pulls, it is always attractive. There is no antigravity, no "negative"  mass. I also learned that most matter is dark, invisible. We know that because we see visible matter moving towards invisible bundles of dark matter and other effects like accelaration or gravity lenses.

What I ask myself is: Couldn't there be "negative" bundles of mass just the other way that pushes matter away instead of invisible dark matter that pulls it? Has someone tried to find something like that?
What is the proof for the fact "Gravity is always attractive" beside that fact that it seems to be?

Granted, it may be a stupid question, but I lack a bit in physics education that is not very related to chemistry.


texts: How dark would the sun become if we traversed a Bok globule?

texts:
Bart Bok gave his name to dust clouds of 2-50 solar masses, which by their darkness may be 100 times more frequent than telescopes can detect.

If the solar system traversed a Bok Globule, how dark would the sun become towards the surface of the Earth, as visible by humans?


texts:
Is there any catalog or any paper published in any journal that lists the stars discovered under whichever star cluster of the Small Magellanic Cloud (SMC) they belong to? There is one for Large Magellanic Cloud (LMC) in the paper Efremov 2003 "Cepheids in LMC clusters and the period-age relation". But I can't find any for SMC.

It would be better still if the stars happen to be Cepheids.


texts: How fast does a spacecraft have to be to enter a primordial black hole without being torn apart?

texts: If there really is a primordial black hole beyond the Kuiper belt, we can send a probe to the black hole and into it. But how fast must the probe be in order to enter the black hole without being ripped apart by spaghettification? The black hole in question has an event horizon diameter of 2-3 inches and 5-15 Earth masses so it is a tiny probe (tiny enough cameras do exist) we would have to launch. Let's say the probe is 1.5 inches broad and has a length of 2.5 inches. How fast must it be in order to not be torn apart and successfully enter the black hole?


texts: Time of day of the K-Pg asteroid impact?

texts: Is it possible to determine what daytime the Chicxulub impactor striked the Earth 66 mya? Was it day or night, what was the current phase of the Moon etc.? Can sediments, rhytmites or something else hold this information?


texts:
Using the library OrbitTools I can input a TLE and get back ECI coordinates. I want to find the RA DEC though, so I use

right_ascension = atan(y / x) * 180.0 / PI;

declination = acos(z/ sqrt(x* x + y * y + z * z)) * 180.0 / PI;

I figure there is likely some constant I need to add (PI/2, PI, etc.) but comparing the calculated RA to that in the TLE, there is no constant that would fix it.

I also noticed x,y,z can be very low (on the order of ~100 km in some cases) which would suggest its measured from earth's surface, so I would need to add the earth's radius to x,y,z, but this also doesn't fix my problem.

Alternatively, I tried using the built in look angle function from OrbitTools, assuming that (0,0,0) (latitude, longitude, height) would be equivalent to RA DEC but... its not. Thanks!


texts: How much solar UV radiation would someone get who were on the surface of Titan?

texts:
The Saturn system is about 10 times as far from the Sun as the Earth. This question is concerning Titan for two reasons: Titan has an Earthlike atmosphere so you don't have to account for a higher radiation due to vacuum. Titan's surface pressure is 1.45 atm (21.5 psi) but imagine you're standing on the summit of a mountain where the pressure were 1 atm (14.7 psi). The 2nd reason is that due to Titan's Earthlike atmosphere you don't need a spacesuit but an oxygen mask and warm clothes only (that should cover your entire body if you don't wanna get frostbites).
Compared to the average UV dose on the surface of the Earth, how much radiation would you get on a mountain peak on Titan? Since Titan is 10 times farther from the Sun, does this mean you get 10 times less the radiation on Earth, or does it decrease with the square so you get a 100 times less radiation of what you get on Earth? Or something else?
Also, do you know a calculator that can determine UV radiation and account for all factors?


texts: Energy Source for Gamma Rays from Merging Neutron Stars

texts:
When the two neutron stars merged in the latest event, large amounts of gamma rays were emitted.  Given that their deorbitting is a result of gravitational wave production, where does the energy for the gamma rays come from?

The only sources I can think of are the rotational kinetic energies of the merging stars or the potential energy released by the final collapse into a single spherical mass.


texts: How strong would gravity need to be to prevent star formation

texts:
This question got me thinking.

In another universe, with a different value for the gravitational constant G, how much bigger would G need to be for star formation to be prevented?

In other words in a hypothetical universe, the star would attempt to ignite, but the force of gravity would overwhelm the star's outward pressure and you would just get a black hole instead.

I'm not looking for a precise answer here... just ROM... 10 times bigger? 10,000 times bigger?


texts:
if the universe is constantly expanding then the distance between black holes and neutron stars must be increasing. Then

What makes them to come under the influence of each other's gravitational field and start rotating, or are they rotating around each other from birth?
Why is the rotary motion spiraling one instead of just rotating around each other in a stable fashion like planets around sun?


texts: Finding the Uncertainty in a Spectral Line

texts: Currently, I am doing a lab in school on atomic spectroscopy. After taking readings of strong emission lines of various elements, I have fit the data using a Voigt Profile function from lmfit. The function minimizes the reduced chi-squared  and returns the best fit along with fitting parameter values (center, amplitude, sigma, gamma). I am mainly interested in the center of the line profile. But, what is the uncertainty in this reported center? Or how would I report the measurement and its uncertainty.


texts: What is the antonym of "closest approach"?

texts:
The distance from Earth to Mars, during their closest approach, is about 55 million kilometers.
At their furthest apart, that distance would be about 401 million kilometers.

Distance
at closest approach
at ___________

Earth–Mars
55 million km
401 million km

Earth–Jupiter
629 million km
928 million km

Is there a standard astronomical name for "their furthest apart"?
My vague understanding is that if one body orbits around another, then we have words like "at perigee" (closest approach to Earth) "at apogee" (furthest apart from Earth); "perihelion" and "aphelion"; and in general "periapsis" and "apsis." However, it doesn't seem correct to talk about the distance from Mars to Earth "at apsis" because neither body orbits the other.
IIUC, in the specific case of Earth and Mars, we might also say that their closest approach happens (more or less) when Mars is in opposition (relative to the Sun, as seen from Earth); but I don't think I can infer from that any useful terminology for their maximum distance — which would happen more or less when Mars is in opposition relative to the Earth, as seen from the Sun. [eshaya's answer indicates that the phrase I'm looking for here is "...when Mars is in conjunction (relative to the Sun, as seen from Earth)." Contrariwise, for Venus, both extrema occur during conjunctions.]
(I'm interested in names for the distance and/or the position. "Closest approach" applies to both distance and position; "apsis" applies only to the position AFAIK. You wouldn't say "the apsis of the Earth and the Moon is 406,000 km.")


texts: Can NASA's sound recordings of planets, and photos of astronomical bodies, be used freely in for-profit projects?

texts:
Can NASA's sound recordings, and photos of, astronomical bodies be used in for-profit/commercial projects (movies, video games)?

I've read NASA's Media Usage Guidelines, but am still not sure.

(here's the link https://www.nasa.gov/multimedia/guidelines/index.html)


texts:
The planets approximately orbit the sun, but a more precise statement is that the planets and the sun all orbit around a common barycenter.

Because the sun is so massive, this barycenter is very close to the sun.  If we take the sun to be the origin, the barycenter travels in a complex, nonlinear trajectory (from Wikipedia):

Modeling this accurately is critically important to predicting celestial events.  I wrote a simulator to predict the 2012 transit of Venus, based on estimates of orbits and the assumption that the sun is at the origin.

The error introduced by these assumptions was enough to miss the transit by a solar diameter.

This data must exist, since the transit actually did happen.  Additionally, errors in the orbital data of the planets could have affected it.  Where can I find data on where the planets and the sun are and what trajectories they're moving in (relative to the solar system's barycenter)?


texts:
In layman terms:

nothing ever escapes the pull of a black hole, not even light
when a super massive star reaches the end of it's life you get a supernova
sometimes the "remains" of these stars can turn turn into black holes

My question is: can a black hole "supernova" the same way a large star does?

What I'm saying is, could this process occur in reverse? I.E:

  black hole -> supernova -> particles form dust cloud -> new star

EDIT: Apologies if any of this sounds stupid (or vague), I'm just curious.


texts: Venus presents same face toward Earth at closest approach. What do you make of that?

texts:
Is reference 6 in this Wikipedia article "dubious" and this passage in need of excising or clarifying?

  Venus's 583.92-day interval between successive close approaches to Earth is equal to 5.001444 Venusian [sic] solar days, making approximately the same face visible from Earth at each close approach. Whether this relationship arose by chance or is the result of some kind of tidal locking with Earth is unknown.

Gold T., Soter S. (1969), Atmospheric tides and the resonant rotation of Venus, Icarus, v. 11, p 356-366

Is it still an "unknown" or "crazy idea that didn't develop with further investigation"?

The idea that Venus is tidally locked with Earth sounds far-fetched. We understand the slowing of Earth's rotation, and I don't recall ever reading of Venus's resistance to the change!


texts: Instead of using flexible mirrors, could software be used for adaptive optics?

texts: Telescopes that use adaptive optics now use flexible mirrors to adjust for distortions in the atmosphere.  The mirrors are flexed in real time to account for the distortions.  I am wondering if it would be feasible to instead use fixed mirrors and use software to make the needed corrections to the image?


texts: Is TESS replaceable by gound-based-small-telescope arrays?

texts:
TESS is designed to perform an all-sky transiting exoplanet survey.

  TESS's survey will focus on nearby G, K and M type stars with apparent
  magnitudes brighter than magnitude 12. Approximately 500,000 stars will
  be studied, including the 1,000 closest red dwarfs.

Since its targets are very bright and its sensitivity is relatively worse than Kepler, I wonder whether it can be replaced by gound-based-small-telescope arrays. There can be no gaps in the time series too.


texts: Where on Earth could dinosaurs feel the impact of their extinction asteroid?

texts: Did the possible Chicxulub asteroid impact around 66 million years ago that made the dinosaurs and many other species extinct, cause the Earth to wobble? How large was the area, where they could "feel" the impact's consequences?


texts:
Phys.org's ALMA sees most distant Milky Way look-alike describes the image reconstruction of a strongly and very nicely lensed z = 4.2 galaxy by a by a foreground galaxy at z = 0.263 and says:

"What we found was quite puzzling; despite forming stars at a high rate, and therefore being the site of highly energetic processes, SPT0418-47 is the most well-ordered galaxy disc ever observed in the early Universe," stated co-author Simona Vegetti, also from the Max Planck Institute for Astrophysics. "This result is quite unexpected and has important implications for how we think galaxies evolve." The astronomers note, however, that even though SPT0418-47 has a disc and other features similar to those of spiral galaxies we see today, they expect it to evolve into a galaxy very different from the Milky Way, and join the class of elliptical galaxies, another type of galaxies that, alongside the spirals, inhabit the Universe today.

and links to Rizzo et al. (2020) in Nature: A dynamically cold disk galaxy in the early Universe. Also see (YouTube and ESO)
Question: Why is SPT0418-47 ("the most distant Milky Way look-alike") expected to evolve into an elliptical galaxy? Is there something about this particular observation that indicates that, or is that just what galaxies "back then" did, even if they had a disk-like phase?


texts: Which star / galaxy is moving away from us the fastest?

texts: I know that we have measured the rate a lot of stars and galaxies move away from us using the doppler shift, and I know that the further a star / galaxy is the faster they accelerate away from us due to space inflation. I'm wondering, did anyone categorise which star / galaxy is moving towards us / away from us the fastest? How fast are they going in relation to our solar system?


texts: What is precipitable water vapor in millimeter-wave radioastronomy and how is it measured?

texts:
Looking through ALMA on-line documentation and articles, the quantity PWV (precipitable water vapor) is a central theme. I have two linked questions.

What (actually) is precipitable water vapor, and is there a "non-precipitable" component also? Why not just call it "water vapor"?
How is it actually measured? It seems to be monitored by radiometers, but I don't understand how that can be used.

I usually see numbers of the order of 5 millimeter to 10 millimeters for very high altitude locations like Atacama. I assume this is the column-integrated amount, expressed in units of liquid, is that right?


texts: Likelihood of extra planets in systems identified by Kepler?

texts:
Kepler has found some systems with multiple planets and shown that it's likely that 7% of sun-like stars have Earth-like planets (video).  I don't think we know enough about extra solar planets to know how planetary systems are organized on average, but the multiple-planet systems are few and would be ones where the orbits of the planets are close to the star and/or flat.  In our own solar system for instance in the small strip of the sky where an observer could see the Earth transit, there are only two small sections where they could also see Venus transit, and if they did happen to see both the Earth and Venus, they would not be able to see any of our other planets transit (up to Saturn at least).

I wrote a small program to show roughly the visible area of sky where each planet could be seen to transit.  Shown is 4 degrees above or below the ecliptic:

Is there just not enough known about the variability of inclinations in orbits of planets in general to speculate about what percent of confirmed candidate planets might have siblings with different inclinations that we cannot see?  If other planetary systems had varying inclinations in their orbits like ours, it looks like for every 'Earth' they found, any other planets would only have about a 7% chance of also being visible.  Is there any reason to think that other systems with multiple planets might have less or more variability in the inclinations of the orbits of their planets?


texts: I had a question about the following chart. What mathematical observation or equation could be made about the data given in respect to the doppler effect? I'm confused if this means that planets are moving at an exponential rate away from us? Would these galaxies appear blue shifted? Thank you in advance for any sort of help.


texts: What's an order-of-magnitude main sequence star look like?

texts:
I'm looking for a very rough, order-of-magnitude approach to the main sequence.

What I mean is, I have a spherically-symmetric hydrogen distribution. I'm looking to get a rough approximation for density $\rho(r)$, temperature $T(r)$, and pressure $P(r)$ given the total mass $M$.

To do this, I need an equation of state. Then I need an approximation for the rate of fusion, which I can balance against the heat lost due to radiation (for which I can use Stefan-Boltzmann).

So, how can I throw this into a back-of-the-envelope calculation that tells me that, e.g. solar mass stars should live ~10^10 years, without crunching on some differential equations?


texts: Does the Sun spread hard radiation waves around the Solar System?  If so, why are we safe here (or are we even safe)?


texts: If Earth is tilted, why is Polaris always above the same spot?

texts: Why is Polaris, the North Star, always above (or near) the North Pole? If Earth is tilted, Polaris' path should be in winter 23 degrees away from its path in summer, or not?


texts: How was lunar eclipse proven using the scientific method?

texts:
I read a few articles about lunar eclipses and while they explain the science behind them, they do not describe who explained it and how it was explained using the scientific method.

Some of them refer to Aristotle 2000 years ago; however, I that didn't really take any measurements using telescopes.

Has anyone in history ever conducted measurements/experiments to "prove" lunar eclipses?


texts: Can the supernova remnant SN 1572 be observed by amateur astronomers?

texts: The SN 1572 remnant, also called Tycho's supernova remnant, is beautiful in X-ray images. It seems to be rather dim in visible light. Are there any amateur photographs of this object? How long are the exposure times needed to get a nice picture?


texts: What happens to the momentum of a star as its mass is converted to energy?

texts:
Stars move relative to many frames of reference.  We can use the CBR for this question, I suppose, but it may not matter.  Their momentum p=mass x velocity.

Stars lose mass as it is converted to energy. Percentage-wise it isn't much, but it is tons per second, so it isn't negligible.
What happens to the momentum of the star as it loses mass? Presumably it is conserved, so does the velocity of the star increase?  Or does the overall system carry off some momentum with the energy, leaving the stellar velocity constant?


texts: Equations for coordinates of solar system objects

texts: I would like to write a program that visualizes the solar system. To do this, I would need equations for calculating the coordinates of each solar system object at a given time. Where could I obtain such equations? I hope they would be as accurate as possible. If there are such equations, it would be also nice to see the raw data they are based on and a description how they were computed. I would be grateful for all help.


texts: Orbital speed is (vector) sum of tangential and normal speed?

texts:
Orbital speed definition in wiki does not state clearly - it is just tangential speed component or square root of squares of normal and tangential speed (full speed vector).

When we say that moon orbital speed is 1 km per sec, we don't know its tangential speed (speed along its trajectory), right? We need some complicated math to calculate it (because of normal speed component inside that 1 km/sec orbital speed)?


texts:
I'm writing an SF story in which a main belt asteroid is diverted into a collision course trajectory with earth (long story), and unknown to us here on earth.

Its H magnitude is 12. As it nears earth, I would assume it gets brighter in V--apparent magnitude. Considering all the eyes on the sky these days, how bright, apparent mag V, would it get before being discovered here on earth? 9 or 10 seem reasonable?

And if you could give its distance in AU, at discovery, that would be super (trying to find out how much time we have left ;) )!


texts: How do tadpole, horseshoe co-orbital states arise and how are they stable?

texts:
A recent paper in Nature "Planetary science: Reckless orbiting in the Solar System" (Morais & Namouni, 2017) presents the following series of four co-orbital states:

While I understand the development of the quasi-satellite and trisectrix states, I am unable to inuit the development of the tadpole and horseshoe co-orbital states.
From explanation provided in the article:

The tadpole and horseshoe shapes arise because the planet's
gravitational attraction alters the body's orbital path — the body
goes through a cycle of catching up with the planet and falling
behind, seeming to change direction from the perspective of the
planet.

How, when approaching the planet, does the body "fall behind" instead of continuing to accelerate toward the planet?
The only means that comes to mind is if the planet orbits faster than the body, but then I imagine it would

No longer be considered co-orbital (because it would have a different orbital period), and
Would eventually be overtaken by the planet.

How, then, do these first two co-orbital states work?
Citation: Nature 543, 635–636 (30 March 2017)
doi:10.1038/543635a


texts:
Has any official information been published regarding the estimated age of the Occator crater on Ceres?  I ran some quick searches but couldn't find anything putting a definitive (or even speculative) date on the moment of impact.

I ask because based upon my own very amateur analysis of the imagery:

...there seems to be a marked decrease in the amount of secondary cratering visible within a fairly uniform radius of the main crater.

My assumption is that when the impact occurred, the surrounding area was likely blanketed by material kicked up from the impact site, obscuring most pre-existing craters (smaller ones, in particular).  Kind of like fresh snowfall, except made of rock.

So for a (small) secondary crater to be visible near Occator, the impact would have to have occurred after the impact that created Occator crater.  Meaning that the relative lack of small craters near Occator would seem to imply a fairly recent impact ("recent" in terms of Ceres geological timescales), I believe?

Is there any official data that would confirm or refute this?


texts: Angular magnification of three lenses telescope?

texts:
What is the magnification of a three-lens telescope?
As a child I built a "telescope": objective lens $$f_1 = 20 cm$$ middle lens $$f_2=5cm$$ and eye lens also $$f_3=5cm$$
My drawing of my telescope:

I measured the magnification "with my eyes". It was about 10x-15x.
But what's the formula?
I had found a formula for a terrestrial telescope, but it doesn't really help. It is said that the middle lens is "only used to correct" the image. Then the magnification should be done like a "normal" telescope (see link):
$$
f_ {objectiveLens} / f_ {eyesLens}
$$
In my telescope, the magnification was dependent on the distance between the middle and eye lenses: larger distance -> bigger magnification.
UPDATE 1:
My idea was calculate magnification of objective like
$$
25/f_{objective}
$$
Then calculate magnification for system of middle and eye lenses like for microscope (here: link). And then multiply both magnifications.
Here is my calculations (see drawing):
Magnification of objective:
$$
m_{objective} = 25cm/f_{1}=25cm/20cm=1.25
$$
In my case distance beetwen middle and eye lenses was 20 cm, then L:
$$
L = d_{2}-f_{1}-f_{2}=20-5-5=10
$$
$$
m_{middle+eye}=(-L/f_{1})*(25/f_{2})=-10/5*25/5=-2*5=-10
$$
Then total magnification is:
$$
M_{total}=m_{objective}*m_{middle+eye}=1.25*10=12.5
$$
But i feel i am very wrong.
UPDATE 2:
I have learn a little bit about lenses and i have use lens equation for calculating angle magnification. But i am not sure that's right. Can someone say something about?


texts: Why have brown dwarf classes been dubbed L, T and Y?

texts:
The classes used to categorize stars (O, B, A, F, G, K, M) are in a bizarre order for historical reasons. Stars were labeled based on the spectral lines that were visible, then the categories were put in a different order to account for their temperature. When discovering the temperature of stars, the categories were not renamed, so that the old catalogs could still be used, and everyone just learned "Oh be a fine guy/gal kiss me".

The first seven classes were updated as knowledge improved, but brown dwarfs were discovered more recently, so why have the letters L, T and Y been chosen, rather than the easier to remember P, Q and R (or some other set of  letters), for instance?


texts:
Following the publication in 1716 of Edmond Halley's method to determine the solar parallax with the transit of Venus, many expeditions were launched to observe the 1761 transits. The following transits of 1769 and of the 19th century were also observed by many astronomers from all over the globe.
The telescope was already invented when the Venus transits of 1631 and 1639 occurred, so astronomers might have been able to observe them. Is there any record of such observations? Were any transits of Venus observed before the invention of the telescope?


texts: What are the RAs of the boundaries between traditional zodiacal signs?

texts:
I'm trying to figure out at what right ascension (RA) — not celestial longitude — the sun enters the various traditional (Western) astrological signs of the Zodiac; but I can't figure out how to calculate this.

What are the right ascensions of the boundaries between traditional zodiacal signs along the ecliptic?


texts: Orbitally convenient Mars-to-Earth transit times?

texts:
Are there any peculiar solar system layouts that would result in highly efficient transits between Mars-to-Earth? I am aware of a Hohmann transfer orbit, but can other planets be used to "sling-shot" large quantities of cargo from Mars to Earth in an efficient manner?

If these cannot be answered directly, what would be the best way to go about calculating interesting orbital positions within the next 1000 years?


texts:
Comments below What is the maximum distance measurable with parallax? discuss challenges associated with parallax measurements of Betelgeuse and link to Wikipedia’s Betelgeuse; Distance measurements which contains the intriguing paragraph:

In 2008, using the Very Large Array (VLA), produced a radio solution of 5.07±1.10 mas, equaling a distance of 197±45 pc or 643±146 ly.83 As the researcher, Harper, points out: "The revised Hipparcos parallax leads to a larger distance (152±20 pc) than the original; however, the astrometric solution still requires a significant cosmic noise of 2.4 mas. Given these results it is clear that the Hipparcos data still contain systematic errors of unknown origin." Although the radio data also have systematic errors, the Harper solution combines the datasets in the hope of mitigating such errors.83 An updated result from further observations with ALMA and e-Merlin gives a parallax of 4.51±0.8 mas and a distance of 222 (+34/−48) pc.[10] Further observations have resulted in a slightly revised parallax of 4.51±0.80.10
83Harper et al. (2008) A New VLA-Hoppocaros Distance to Betelgeuse and its Implications
10Harper et al. (2017)A Updated 2017 Astrometric Solution for Betelgeuse

Looking at section 3.1 of Harper et al. (2008):

As discussed in Section 1, Betelgeuse required a significant additional error source (cosmic noise) to obtained the expected χ2. The actual nature of this cosmic noise is not known but it may be related to photocenter movement which is unlikely to be random in position on short timescales and would likely provide a systematic error in the astrometric solution. The position angle of the star's rotation axis has been measured from spatially resolved ultraviolet Hubble Space Telescope spectra. Uitenbroek et al. (1998) found ~55° (measured East of North) from absorption features in Goddard High Resolution Spectrograph data, and Harper & Brown (2006) found ~65° from emission features in multi-epoch Space Telescope Imaging Spectrograph data. The distribution of photocenters, perhaps driven by convective and Coriolis terms, may have a special relation to the rotation axis. The stellar proper motion vector also has a position angle of 68° and brightness fluctuations that occurred preferentially near the stellar equator might induce scatter in the parallax displacements, although fluctuations due to stellar rotation itself, would be unlikely because of the ~17 yr rotational period (Uitenbroek et al. 1998).

If I understand correctly; the problem with measuring Betelgeuse's position accurately is that it is so large and non-uniform that its "photocenter" (radio, optical, or otherwise) can be significantly different than its center of mass.
And the problem with obtaining accurate parallax determinations is that its non-uniformity changes on timescales comparable to a year. If it were much faster or much slower, it would not be so much of a challenge.
Interferometric optical imaging of Betelgeuse confirm dramatic non-uniformity across its disk.
Question: What will it finally take to accurately measure the distance to Betelgeuse?

To see how nonuniform Betelgeuse appears optically: (feel free to edit and add better links)

this answer to What else could the Event Horizon Telescope Observe?
this answer to How far have individual stars been seen by radio telescopes?
Why can't the surfaces of stars be observed? links to Wikipedia's List of stars with resolved images
This answer to Betelgeuse in false colors, is this common?


texts: How did the authors determine both the spatial size of gas cloud HCN-0.009-0.044 and its central mass at the same time?

texts:
Phys.org's Hiding black hole found says:

A research team led by Shunya Takekawa at the National Astronomical Observatory of Japan noticed HCN-0.009-0.044, a gas cloud moving strangely near the center of the galaxy 25,000 light-years away from Earth in the constellation Sagittarius. They used ALMA (Atacama Large Millimeter/submillimeter Array) to perform high-resolution observations of the cloud and found that it is swirling around a massive invisible object.
Takekawa says, "Detailed kinematic analyses revealed that an enormous mass, 30,000 times that of the sun, was concentrated in a region much smaller than our solar system. This and the lack of any observed object at that location strongly suggests an intermediate-mass black hole. By analyzing other anomalous clouds, we hope to expose other quiet black holes."
Tomoharu Oka, a professor at Keio University and coleader of the team, adds, "It is significant that this intermediate mass black hole was found only 20 light-years from the supermassive black hole at the galactic center. In the future, it will fall into the supermassive black hole, much like gas is currently falling into it. This supports the merger model of black hole growth."
These results were published as Takekawa et al. "Indication of Another Intermediate-mass Black Hole in the Galactic Center" in The Astrophysical Journal Letters on January 20, 2019.

The paper Indication of Another Intermediate-mass Black Hole in the Galactic Center (open access) is pretty hard to read as it details the careful analysis of ALMA data reduction and analysis.
Question: How did the authors determine both the spatial size of gas cloud HCN-0.009-0.044 and the mass of the central object at the same time?


texts: Succinct explanation of black hole mass, diameter, shape?

texts:
I don't have any formal education in physics and I have found it hard to find a succinct and somewhat simple to comprehend explanation of what we think is the actual reality of a black hole. I'm asking after seeing a statement saying that the mass of the central supermassive black hole in the Phoenix Cluster was 20 billion solar masses and the diameter we think to be approximately 118 billion kilometres across.

Is someone able to point me to a resource or give me a simple explanation as to what we think a black hole actually is in shape? Is it a solid sphere with a central point and a radius of approximately 59 billion kilometres? A spinning disc with a face that is 118 billion kilometres across and if so what are the other measurements?

Am I thinking about this all wrong with regards to trying to understand it as a shape at all? If I understand correctly, mass doesn't give any indication of size as it is dependent on density. Is a black hole just basically a somewhat spherical body of mass like any other that just has an absolutely massive density and spin?


texts: What's the difference between grism and grating?

texts:
In spectroscopic observations, sometimes you meet grism, sometimes grating.

Both of them could cause light dispersion, but what is the difference?


texts: Are there any openly available simulators describing how planets form from protoplanetary disk?

texts: Out of pure interest, I wonder whether there are existing open-source simulators that can simulate the formation of stars and planets from a protostar and the surrounding protoplanetary disk?


texts: Is there a theoretical minimum mass main sequence star that can become a red giant?

texts:
Wikipedia's Red Giant page says they range in mass from 0.3 to 0.8 solar masses.   Which corresponds to an initial mass of a bit less than 1 solar mass, about 0.8 or so, if I was to estimate and a few solar masses.   Given that over half the mass is lost prior to the Red Giant stage.

Any stars with less than 0.8 solar mass to begin with wouldn't have had enough time to go red giant, so it's possible that significantly smaller stars could eventually go red giant.   Is there an estimate for how light the main sequence mass can be where the star still becomes a red giant late in it's life?


texts:
I'm looking for visualizations that show the movement of stars in barred galaxies but not having much luck. I've found many that simulate spiral galaxy arms in general, but none that include a bar or a nucleus.

I'm trying to word my question in a way that is not dependent on my own assumptions, but some of my assumptions are:

the area that includes the bar is the nucleus.
the nucleus and the bulge are not necessarily the same thing. (Bars within the nucleus is a result of gravity waves. Bulge is an artifact of how the galaxy was formed.)
the edge boundary of the nucleus is a gravitational "tipping point".
stars are not crossing this boundary (ie: they either belong to the nucleus or to the arms, not both).

What is the movement of stars within the nucleus of a barred spiral galaxy, and how do their galactic orbits differ from stars outside the nucleus?


texts: How can absolute value of azimuth exceed distance from pole?

texts:
I'm sure I'm just doing something dumb, but:

Yildun has a declination of approximately 86.5. This puts it 3.5
degrees from the celestial north pole.
Per the snapshot below, Yildun's azimuth can be as high as 4.17
degrees.
Since the celestial north pole always has an azimuth of 0,
wouldn't this put Yildun 4.17 degrees from the pole, contradicting
the distance calculated from declination?

What am I missing?

EDIT: I thought I'd figured this out:

On Earth, one degree of latitude is not the same as one degree of longitude (except at the equator).
Analogously, in the sky, one declination degree is not the same as one right ascension degree.

I thought the situation for right ascension applied to azimuth, but it doesn't.

Both azimuth and declination are measured on "great circles" (declination is measured on a half circle, but same general idea), so degrees in azimuth should equal degrees in declination.


texts: Gravitational lensing in Newtonian physics

texts:
Famously, when Eddington attempted to measure gravitational lensing during the Eclipse, it was the measured magnitude of the lensing that gave gravity [pun, obviously, intended] to General Relativity - not the measurement of lensing itself. That is, Newtonian physics also predicted a lensing deflection, but only half of the deflection predicted by GR.

Question is: Why? I've read a lot about this, and I can see how when one integrates the Newtonian acceleration along the photon path with constant |v|=c this gives the 'correct' Newtonian value - but intuitively, I can't wrap my head around it. Why should a massless photon be affected by gravity at all in Newtonian gravity?


texts: tidaly locked celestial body in three planet system

texts: If a Planet have 2 Moons (so suppose system of 3 bodies) and we say "One Moon is tidally locked to Planet" how can we say that it is tidally locked when Baricenter of 3 bodies has its trajectory?


texts: Is it possible to explain the difference between synodic month and sidereal month by degrees?

texts:
Is it possible to explain the difference between synodic and sidereal month by degrees - in the following way?

Just recently I learnt about these two types of months, and I want to see if I can understand the difference by degrees method. Assuming the orbit's moon is 360 degrees, the sidereal month starts from the point where 1st degree to (include) 1st degree, while the synodic month starts from 1st degree to the 0.5 degree (after full orbit's moon, because the angular size of the moon is 0.5 degree). Is it possible to understand why it that's way?


texts: The formation of absorption lines in solar spectrum

texts:
It is said that absorption lines originate from regions higher in the photosphere where the gas is cooler.

The gas should be absorbing photons, then re-emitting them... absorb, re-emit, absorb, re-emit...

My question is where are the photons produced by re-emission? Why are there absorption lines?

Although photons could be scattered, photons at other places could be scattered to our line our sight. Surely the total number of photons does not drop the further out you go? So while photons originally headed in the direction of an observer are possibly absorbed and re-emitted in a direction away from the observer, surely this will be made up for by a photon being emitted from another location in the direction of the observer? Otherwise wouldn't there be a location somewhere else where a bunch of extra scattered photons can be observed?

If we integrate all the photons at the lower surface of the photosphere A and integrated all the photons at the upper surface of the chromosphere C, surely there are the same number of photons? Otherwise, where have they gone? Where is the lost energy?

Are there less photons emitted from the surface C because some have been emitted back down into the Sun? But that has to reach an equilibrium, or there would be a cascading increase in the number of photons inside the sun.


texts: Given a star's ra, dec how can I compute one pair of lat, long and time where the star would be at zenith?

texts:
As the questions says, I have the celestial coordinates of a star(ra, dec). My trigonometry is not so great so I haven't managed to figure out a solution to this. I want to find a place (lat, long) and a time when the star will be right above the computed point?

I.e. star coordinates: 7h 23m 23.97s -13° 25' 2.64", where would be the point(lat, long) and time where the star would be right above?


texts: How many observations does it take to determine the orbit of a TNO

texts:
Several observations of a distant solar object must be taken before determining its orbit, but in the case of objects discovered beyond Neptune's orbit, just how many are required over what period of time (or what portion of arc) before a reasonably accurate orbit can be determined, sufficient, say, to be added to the Minor Planet Database or a JPL ephemeris?

I'd be interested to know to what degree of certainty orbital elements for objects beyond 30 AU can be determined when we have only observed them for (so far) short portions of their orbits. What about 60 AU, or 90 AU?

As an example, the Wiki for VNH004, a TNO that is a photo target for the New Horizons probe, states:

  The asteroid only was observed 12 times by the Mauna Kea and Las Campanas Observatory over a period of about 33.8 days between May 29th and July 2nd, 2011, so its current orbit is extremely uncertain.

How uncertain? Here are orbital elements along with +/- values:

  Heliocentric elements and errors
  Epoch . . . . . .  2455710.5000  =  2011/05/29
  Mean Anomaly: . . .       309.75557 +/-    31.234
  Argument of Peri: .     112.06593 +/-    54.010
  Long of Asc Node:  .    247.74234 +/-     3.336
  Inclination: .  . . . . . . . .   3.81106 +/-     0.396
  Eccentricity: . . . . .     0.29886369 +/-    0.8050
  Semi-Major Axis: .    37.47217640 +/-   14.2156
  Time of Perihelion: 2467404.0720 +/-    2926.4
  Perihelion: . . . . . . .   26.27310354 +/-   31.7695
  Aphelion: . . . . . . . .    48.67124926 +/-   35.3678
  Period (y) . . . . . . . . . . 229.3885 +/-    130.53

An orbital period of over 229 years, plus or minus over 130 years. How many more observations would be required to half the margin of error?


texts: GW from merging of neutron stars and black holes

texts:
If it had been detected gravitational waves from merging of black holes (BH), and neutron stars (NS), why there are no detection of black holes merging with neutron stars?

Why a BH-NS merging should be a rarer event than BH-BH and NS-NS?


texts: What is meant by "vastness of space, which now filled a volume of a hundred million light years"?

texts:
I am reading the book Cosmic biology: How life could evolve on other worlds (citation below) and do not understand the meaning of the following paragraph from page 4 (emphasis mine):

It took 200,000 years or more for the universe to cool to 2700 °C, a temperature at which the protons and neutron-proton aggregates could start capturing the free flying electrons to form the first atoms of hydrogen and helium.  As the dense fog of electrons condensed into newly formed atoms, the opaqueness cleared and photons could stream without interruption across the vastness of space, which now filled a volume of a hundred million light years.

I am under the assumption that a light-year is a unit of measurement for distance and spatial volume is expressed as though it fills a cube (e.g., 100 ly^3).  Even if this practice of denoting volume was unintentionally omitted, the claim does not satisfy my attempt to calculate the spherical volume:
Given 200,000 years of elapsed time since Big Bang -> photons could have traveled a distance of 200,000 ly in any direction.
Formula for radius of a sphere: $$\frac{4}{ 3} \pi r ^ 3 = \frac43  \pi  (200,000 ^ 3) = 3.35\times 10^{16} ly.$$
This result is much larger than "[one] hundred million light years."
Am I misreading this paragraph?

Irwin, Louis Neal, and Dirk Schulze-Makuch. Cosmic biology: How life could evolve on other worlds. Springer Science & Business Media, 2010.


texts: Techniques for locating origin of gravitational waves

texts:
The gravity wave detections (GW150914 and GW151226) have both been ambiguous in their exact position, the LIGO papers (GW150914 and GW151226) give no precise information about the location of the origin of the waves.

In the case of GW150914, the approximate location is in the area known as the Magellanic Clouds. In case of GW151226, the parsecs are known, but no precise location. There are two detectors (in Hanford and Livingstone,USA) that made gravitational wave detection possible, does one need a third detector(or more) to pinpoint the position of the origin of these waves?

In other words can triangulation from three detectors pinpoint the location  or are there other techniques for pinpointing the origin of gravitational waves?


texts: Is the M81 group bound to the Local group?

texts: Andromeda and the Milky Way are set to collide in 4bn years. Will the Local Group collide with M81, and what about further away groups? if so, when?


texts: Superconducting space dust; what transient fields could induce persistent current loops?

texts:
Science Alert's Superconductivity Has Been Discovered in Meteorites For The First Time quotes from Superconductivity found in meteorites (Wampler et al. PNAS March 23, 2020)
The paper describes the identification of grains within meteorites that have superconductivity transition temperatures as high as 6 Kelvin.
At the end of the discussion the paper explores possible astronomical implications of naturally occurring superconducting dust particles; "...superconducting particles could sustain microscopic current loops generated by transient fields and contribute to nearby magnetic fields."
That section is reproduced below.
Questions:

Are there any known phenomena that can produce "transient fields" that could induce such microscopic current loops that persist after the transient has passed? Would the transient have to be step function or could it be a pulse and return to baseline? Would it need to be electric or magnetic or could it be either?
Have the implications of superconducting space dust been explored previously? (I'm supposing not since the authors haven't cited anything and would have probably done a thorough literature search)

While these nonchondritic meteorites do not carry a history of the interstellar medium (1), the fact that these phases can form naturally in macroscopic grains shows that there exists a natural process that can create these phases. This suggests the possibility that superconducting material phases could also be found in interstellar grains deep within the coldest regions of space, where these phases would be in a superconducting state. For example, cold, dense molecular clouds have typical temperatures as low as 10 K (29), but in regions with particularly low ultraviolet stellar radiation, temperatures are estimated to be as low as 5 K (30) or even colder (31). In unusual parts of space, temperatures can be even lower (32). The superconducting phases detected in these meteorites would be superconducting in these regions. Furthermore, other metallic alloys have been shown to have critical temperatures above 10 K (33, 34), which could exist naturally if they could be created in similar conditions to the phases detected in this manuscript.
Superconducting particles within cold regions of space
could have implications on the structure of stellar objects. Specifically, superconducting particles could sustain microscopic current loops generated by transient fields and contribute to nearby magnetic fields. The origins of some magnetic fields, such as those observed in giant molecular clouds (35), are not well understood. Whether or not naturally superconducting phases would have any significant effect would depend on the critical temperature and quantity of such phases, and therefore deserves further study. There is an aggressive expansion of research and new missions to space, searching for new materials in extraterrestrial objects. This study suggests that the search for naturally occurring superconducting grains and their roles in astronomical objects should be a new component of such research.


texts: How many stars would it take to draw a line across the middle of the sky that appeared solid?

texts: I was talking with a friend about how slowly the star field changes (based on the speed that we are moving through the galaxy) and I started to wonder about a star's visible size.  They are basically the pixels that make up our sky.  This made me wonder, how many stars (let's use the north star as a reference.) would it take (lined up side by side) to draw line that appeared solid across the middle of the sky, perpendicular to the horizon which appeared solid?


texts: How hard is it to find the Sun's "sisters?"

texts:
As @ProfRob stated in his excellent answer regarding the ejection of the Solar System's fifth gas giant,

It is for similar reasons that, even though the Sun was probably born in a cluster of $\sim 10^4$ stars, none of those siblings have been firmly identified.

So why is it so difficult to find such stellar siblings?
NB: Maybe it's because open clusters tend to disassociate quickly and the stars would be scattered everywhere. But wouldn't stars with similar spectra, age, metallicity, etc. be found easily?


texts: What are Native coordinates in the World Coordinate System?

texts:
i am writing my first paper about our solar telescope, and i need to explain the optical transformations in terms of World Coordinate Systems. I am not sure if i got the following right, and would be appreciated if someone could help understanding it(Paper II from https://fits.gsfc.nasa.gov/fits_wcs.html):
Are native coordinates just normal (in my case (az,alt)) spherical coordinates, with its origin in the pole/reference point (in my case, the sun center) of the linear transformation?
Are Projection Plane coordinates measured in arcsec? (i dont need this, but would like to understand: why isnt the projection plane of a camera a sphere?)
Would really appreciate an answer! Have a nice day.


texts: Will a new planet form if Jupiter's influence on asteroid belt will diminish in a few billion years?

texts: I know that tidal forces are pushing Jupiter farther from the Sun, but I couldn't find exactly the yearly amount. In a few billion years would this effect (and subsequent decrease in gravity pull) allow the formation of a new planet from the asteroid belt or would Jupiter take the asteroids with it farther away?


texts:
I know that light is considered as energy, and as such has a mass.

So it is certainly taken into account in universes models and calculations.

So I'm wondering: How much of the total mass of the universe does light and electromagnetic waves accounts for? And how much does it contributes to its expansion?

Is it positive or negative? Since it is considered as mass only, it should decrease expansion somehow. Am I right about this?


texts: Why do the magnetic field lines of the earth go from south pole to north pole?

texts:
I saw these figures and could not get why the magnetic field lines are not pointing from the north pole towards the south pole but they do the opposite.

Or also this picture which is about magnetic characteristics of the sun says the same thing:


texts: Are black holes spherical during merger?

texts: I've been thinking about black holes, specifically during the final moments before two merge.  I'm wondering if black holes, or I guess more specifically their event horizons, are always spherical.  It seems to me that in the moments before two merge, their respective event horizons will be stretched, somewhat like how the Moon causes our ocean's tides. I have drawn a (poor) diagram of what I think they may look like.  Notice how the event horizons are closer to the singularity on the inner side, this is because the gravity from each black hole is in opposition.  The event horizons are further from the singularity on the outer side because the gravity from each black hole adds up.


texts: Do celestial bodies have many asteroids orbiting them?

texts:
Are there statistics on the expected number of asteroids (of various sizes) to orbit planets and moons in our (or other) solar systems?

Do we know of other natural bodies orbiting the Earth (besides the Moon) or asteroids orbiting the Moon?

If not, why not? Though the likelihood of natural captures are low, it is my understanding that there are lots of asteroids (though, of course, all orbits will eventually decay).


texts:
We know Halley's Comet returns every 75-76 years. We can reasonably compute its elliptical orbit.
We know that the Oort Cloud is a cloud of predominantly icy planetesimals proposed to surround the Sun at distances ranging from 2,000 to 200,000 au. We divide it into  two regions: a disc-shaped inner Oort cloud (or Hills cloud) and a spherical outer Oort cloud.
We know that some comets are thought to have originated from the Oort cloud.
My question is: Does Halley's Comet travel past the outer bounds of the Oort Cloud?


texts: do rockets follow a straight path but is looks curved because of space time?

texts:
Does the path of a rocket or anything going straight up look curved because the mass of the earth is curving space-time?

If we assume the earth, the rocket and all the celestial bodies whose gravitational field reaches the earth to be massless then will the rocket follow truly a straight line, because there would be nothing curving the space-time?


texts: How is the Milky Way on a collision course with Andromeda when the universe is expanding?

texts:
At some time in the future it is said our galaxy will collide with Andromeda. Yet at the same time the universe is expanding at a ever-accelerating pace. Using the old raisin-in-the-baking-loaf-of-bread analogy, the raisins grow farther apart and don’t “collide” with each other during the baking process. It seems to be the same way with galaxies and the expanding universe.

How are these two seemingly conflicting postulates reconciled? What am I missing here?


texts:
A couple days ago, I zoomed in with my 30x optical zoom camera, and after some exposure adjustments, a bright star in the night sky turned into this:

Are those other planets or other stars?  Or is that a lens effect?

EDIT: The bright object in question was ~60 degrees above the horizon, and ESE of me (East-south-east). I took the picture on 3-25-16 from Madison, Wisconsin.

EDIT: Question answered, more clear picture added FYI.


texts: Why do stars seem not to move relative to each other?

texts:
Whilst I understand why stars move across our skies,(Earth rotation) I struggle to understand why they do not appear to move relative to each other.

To explain:  Take the best know constellation the Plough.  It is a familiar site to most people.  But it has been there apparently un-changed for years.  If we (I mean earth, moon, stars and everything else ) are hurtling through space, then surely changes should be apparent unless we are all travelling in the same direction at exactly the same speed.


texts: Why is Proxima Centauri called Proxima Centauri?

texts:
It seems very logical, to call the star, closest to earth, "Proxima" ("Proximus" is Latin for "close" and the word for star, "stella", is feminin, hence "Proxima"), but there's one thing I don't understand: in order to call a star ±"The closest one", you must be sure that in future, no closer star will be found.
This is not so simple: lots of very close stars, like Barnard's star, Kapteyn's, ..., are very weak and even not visible to the naked eye.
So, how can we be that sure that Proxima is indeed the nearest one? Have astronomers indeed examined the complete sky in order to prove this?


texts: Measuring declination in negative hours of arc?

texts:
I'm running the program found at https://www.projectpluto.com/source.htm in order to find the Right ascensions and declinations of celestial bodies at various points in time. An odd thing I've noticed is that the declination is reported in hours, minutes, seconds as opposed to degrees, minutes, seconds and I'm not sure how to use that. Especially considering the hours seem to lie on [-24,24] which doesn't make sense for the [-90,90] range for declination measured in degrees.

An example:

  Sun RA=10:11:50.1 Dec= 11:08:50.8

This is sampled from Los Angeles, California at 8 PM PDT (3:00 UTC) so I would expect when I ran these numbers to calculate elevation and azimuth that the elevation would be a good bit under the horizon.

I tried scaling the hours by 90/24 so that it would be back in the [-90,90] range but when I tried running those numbers through this tool, it ended up reporting that the sun would actually be just above the horizon which is definitely wrong.

So with that said, how should I be interpreting this data to get the result I expect? I'm pretty new to this, so I may be lacking something incredibly fundamental.


texts: Could you have an Earth-like planet with a crust thick enough to prevent volcanos from forming?


texts: Where are the two recently confirmed unusual brown dwarfs?

texts: The News report of two confirmed unusual T-type brown dwarfs but they don't tell where in space or on the night sky they were found or how far they are from the Earth/Sun. The brown dwarfs are WISE 0414 and WISE 1810. Does anyone know where they are located?


texts: Is it possible that we see our solar system as a star?

texts:
A few days ago I just read here that our galaxies (one of which we are part of) move or expand faster than light,
And read here that light we see from stars is extremely old, maybe more than several years.

If galaxies' expansion is faster than the light of stars, then is it possible that we see our own Sun as a star somewhere because light may reach us later than expansion?


texts: Invisible in visible light but visible in non-visible light?

texts:
This may be a stupid question, but I've been wondering this for a while and haven't yet found a solid answer. I'm worried the answer will be something along the lines of "of course not, you dummy" but it's bothering me and I haven't found the answer so I figured I'd take the risk.
Is it possible for matter to ONLY be visible in an area of the spectrum other than visible light? For instance, can any matter be invisible to us but visible in the UV or IR range?


texts: How much illumination do the background stars provide?

texts:
I'm trying to get a better understanding of what things look like in "outer space." The main problem I'm trying to solve right now, is how much illumination is provided by the background stars. Is it even perceivable? If we were to ignore the light contributions from our sun and local planets, how difficult would it be to see an object (within arm's reach) that was lit only by the remaining ambient light?

Another way this might be thought of is: how dark is the dark side of Pluto? If a person were able to stand on the surface of Pluto, would a moonless/Charonless night be totally pitch black, or would there be enough ambient light to see? How bright might that be?

EDIT: The example about Pluto is a little deceptive because Pluto will block a large portion of starlight. I'm thinking of the starlight that is neither blocked by nor supplemented by any of the bodies in our solar system.


texts: Could the E.H.T. produce an image of the human artifacts on the moon?

texts:
Two days ago the New York Times featured the Event Horizon Telescope (E.H.T.), a huge virtual radio telescope emerging from combining data from several dishes scattered around the world. While the E.H.T. is designed and built to observe black holes, specifically Sagittarius A*, the article claimed that the E.H.T. could "spot a doughnut on the moon".

This was probably just meant as an illustration of its enormous resolution, but: Could it? I'm aware that the moon is not a radio source but it was apparently possible to make radar images of its surface using reflected radar radiation from one of the involved dishes. Would something like that be possible with the E.H.T.? Could we "see" the landing sites from earth? Left behind rovers, descent and ascent stages?


texts: Corresponding BV Value to a Hertzsprung–Russell

texts:
I have an example BV value from an astronomical catalog of 1.04 (33 Psc, HD Catalog 28).

Given only the number 1.04, can that be charted on the HR, or do I need the individual components of the value and/or temperature and luminosity?

Edit:

I should explain why I'm asking. I've written an application that displays data from one of several catalogs, and they only provide the BV value. I want to add a component to the application that would show an HR diagram with that star superimposed on it. The example of 33 Psc was semi-random, as it's the third entry in one of the catalogs (Yale Bright Star Catalog).


texts: Do the heliospheres of stars at the center of globular clusters interact?

texts: I read that the stars in the center of globular clusters are incredibly close to one another and that these stars can be spaced 100 to 1000 times closer than the stars near the sun. Because these stars are so closely spaced, would their heliospheres interact with each other? There are binary stars whose coronas interact and create powerful x-rays. Would something like that happen with those central globular cluster stars, but on a much larger scale, what with the larger amount of stars involved?


texts: Why can't this be the simple (and obvious) explanation for the dimming of KIC 8462852?

texts:
Couldn't it be just a relatively large object, anywhere in the interstellar space between the star and Earth? I mean, look out the window at a far away car or a tree, then stretch out your hand and you can block it entirely with your thumb. What if the thumb is that object (a planet, a star, a comet, some space debris, etc.) that's between Earth and the star, and it's just the right size to only block that star, and no other stars. Why has this very simple explanation been ruled out (if it has been)?
I know that the debris next to the star explanation has been ruled out because of the lack of IR that would come from that debris being heated up if it was close to the star. But this is not a problem here, if the blocking object is far away from the star.


texts: How early could we detect an asteroid the size of the one that caused the extinction of the dinosaurs?

texts: Would we know months, years ahead? How sophisticated is our technology's ability to detect it at 100% accuracy?